I know very little about trailers, but I am working on one myself. With your concept of the spare....
Do you think that you could put is slightly forward of the axle, and hang it right above the axle? The reason I ask is because I think that would be the least likely place to cause an issue on the trails. If you are going to drag that thing over any rocks at all, well, I have had to slide my axle over rocks that the tires just did not reach the ground anymore. So if you found yourself in that place you would also find the tire pushing against the rocks, getting cuts and knicks, and perhaps even stalling your forward progress. With the SOA lift I should think that the chances of the axle touching the tire would be minimal at best.
Again, speculation, so, sorry if I am wrong there...